This invention generally relates to a device for substantially purifying exhaust gases emitted from an internal combustion engine and, more particularly, to a filter having a fixture supporting heating wires for incinerating exhaust gas particulates trapped in the filter.
As is well known to those skilled in the art, exhaust gases emitted from a diesel engine contain a relatively large amount of fine particulates mainly composed of carbon particulates. For minimizing the emission of such exhaust gas particulates, various purifying devices have been devised.
A simple form of the prior art purifying device is the use of a filter member made of wire meshes or refractory fibers and disposed in an engine exhaust system for trapping the exhaust gas particulates before they are discharged to the atmosphere.
Another type of the prior art devices comprises a filter block made of ceramic material such as cordierite and having a plurality of axially extending parallel channels which are separated from each other by porous partition walls, some of the channels being closed at their inlet ends with blind plugs while the remaining channels are closed at their outlet ends with similar blind plugs. This type is disclosed in, for example, the Japanese Laid-open Patent Publication No. 56-148607, published Nov. 18, 1981, and the U.S. Pat. No. 4,346,557, issued Aug. 31, 1982, and when in use as disposed in an engine exhaust system, operates so as to trap the exhaust gas particulates as the exhaust gases from the engine flow through the filter block in a direction from the unclosed inlet ends of some of the channels to the unclosed outlet ends of the remaining channels across the porous partition walls. However, it has been found that there is a problem in that the filter block tends to be clogged with the exhaust gas particulates as a result of the pile-up of the particulates, when the purifying device is used for a substantial period of time, to such an extent as to result in the increased exhaust gas pressure with the consequent reduction in engine power output.
In view of the above, the use of heating wires such as nichrome wires has been contemplated, as disclosed in, for example, the Japanese Laid-open Patent Publication No. 55-131518, published Oct. 13, 1980, to clean or regenerate the filter by incineration of the exhaust gas particulates trapped in the filter. According to this prior art, the heating wires are bonded to the filter by the use of refractory bonding material and are adapted to be electrically energized to emit heat necessary to ignite the trapped exhaust gas particulates each time the filter has been clogged.
However, according to the prior art disclosed in the Japanese Laid-open Patent Publication No. 55-131518, since the heating wires are bonded to one end face of the filter block by the use of the bonding material, as described above, not only is the use of the bonding material required, but also the heating wires once bonded to the filter block tend to separate readily therefrom, and difficulty is also encountered in firmly fixing the heating wires.
In addition, when the manner in which the exhaust gas particulates pile up in the filter is taken into consideration, it is generally recognized that the exhaust gas particulates entering each channel form a layer adhering to the interior surface of the surrounding partition wall with its thickness maximized at a location slightly inwardly from the inlet end and gradually decreasing towards the outlet end. Therefore, since the device such as disclosed in the last mentioned publication is such that the heating wires are positioned on the downstream side of the filter block with respect to the direction of flow of the exhaust gases from the engine towards the atmosphere across the purifying device, the filter can not be efficiently regenerated.